Wireless underground sensor networks (WUSNs) are networks of wirelessly-interconnected sensor nodes deployed in a variety of underground environments, such as soil, underground tunnels, and oil reservoirs. WUSNs can enable a wide range of emerging applications, such as mine and tunnel disaster prevention, oil gas extraction, underground power grid monitoring, earthquake and landslide forecast, border patrol and security, underground animal tracing, and many more other applications. Most of the applications require the knowledge of location information of the randomly deployed sensor nodes. However, the underground environments prevent the direct application of the conventional localization solutions based on the propagation properties of electromagnetic (EM) waves because of the extremely short communication ranges, highly unreliable channel conditions, and large antenna sizes.